perf sched: Fix thread leaks in 'perf sched timehist'

Add missing thread__put() after machine__findnew_thread() or
timehist_get_thread().  Also idle threads' last_thread should be
refcounted properly.

Fixes: 699b5b92 ("perf sched timehist: Save callchain when entering idle")
Reviewed-by: Ian Rogers <irogers@google.com>
Tested-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250703014942.1369397-5-namhyung@kernel.org


Signed-off-by: Namhyung Kim <namhyung@kernel.org>